Hey conor, as an eBay seller I can tell you what you need to put int the auction description to get more bids:

Let them know if it is an actual signed photograph, a copy or stamped autograph, or a photo copy or scan of a signed photo.

Also, tell them how you got the autograph. People like to relate to the subject at hand. Give them the whole story.

Certificate of authenticity is not neccessary. It is just a piece of paper that anyone can make up. Unless it is a licensed product or film set piece you don't actually need proof. A simple "how I got it" will suffice.